Thermally stabilized cellulosic material produced by treatment with diglycolamine in combination with pentaerythritol



United States Patent 3,403,968 THERMALLY STABILIZED CELLULOSIC MATE- RIAL PRODUCED BY TREATMENT WITH DI- GLYCOLAMINE IN COMBINATION WITH PEN- TAERYTHRITOL Berthal D. Brummet, Cincinnati, and Fred S. Sadler,

Zanesville, Ohio, assignors to McGraw-Edison Company, Milwaukee, Wis., a corporation of Delaware No Drawing. Filed Feb. 25, 1965, Ser. No. 435,342

7 Claims. (Cl. 8-116.2)

This invention relates to cellulosic materials having improved thermal stability, and more particularly to cellulosic materials to be used as insulation in electrical apparatus.

Cellulose fiber materials tend to deteriorate when subjected to elevated temperatures for extended periods of time resulting in a progressive reduction in their strength until eventually they fracture. When used in an electrical apparatus, the deterioration of the cellulose is accelerated by other factors, particularly where the insulation is in contact with, or immersed in, a liquid dielectric such as transformer oils. The elevated temperatures may cause the liquid dielectrics to break down into their chemical constituents and the resultant deterioration products may in turn attack the cellulose fiber material.

The problem of deterioration of cellulose fiber material at elevated temperatures is also encountered when using cellulose fiber cord reinforcing in rubber articles such as pneumatic tires, steam hoses, conveyor belts and the like. In this situation, the cellulose fiber reinforcement, which is subjected to elevated temperatures either from external heat or from heat generated internally by reason of repeated flexing, as in the case of pneumatic tires, tends to deteriorate, resulting in a progressive reduction in the strength of the cellulose fiber reinforcement.

The present invention is directed to a cellulose fiber material impregnated with diglycolamine or a combination of diglycolamine and pentaerythritol. Impregnating the cellulose fiber material with either of these additives increases the thermal stability of the cellulose fiber and enables the fibers to Withstand deterioration by the action of heat over extended periods of time. The fibers are not only protected against thermal deterioration, but are also stabilized against attack from decomposition products of transformer oils or other liquid dielectrics. For this reason the cellulose material treated in accordance with the invention is particularly useful in oil-filled transformers or other similar electrical apparatus.

The cellulose fiber material to be treated can be rayon, paper, cotton, linen or other common cellulosic materials. When used as an insulation material in an electrical apparatus, the cellulose material will generally be composed of rag, kraft or manilla paper.

The active ingredients, either diglycolamine or a combination of diglycolamine and pentaerythritol, are preferably applied to the cellulose fibers in the form of an aqueous solution although in some cases the pure active ingredients can be used. The fibers can be impregnated in any suitable manner such as by immersing the fibers in a treating solution for a time sufficient to fully impregnate the fibers, or by spraying, brushing, dipping, size pressing or the like. It is important that all of the individual fibers of filaments of the cellulose material be impregnated with the treating solution so that each individual cellulose fiber is able to react with the active ingredients. Impregnation differs from surface coating processes in which only the outer surface of the fibrous material is coated with the active ingredient and the individual fibers on the interior of the material are generally uncoated.

The impregnation of the cellulose fiber material can be carried out at room temperature or at elevated temperatures up to the boiling point of the solution employed.

m 3,403,968 1C6 Patented Oct. 1, 196

The time of contact between the cellulose fibers and the solution should be sufficient to permit penetration or impregnation of the fibers. Generally a contact time of 15 seconds to 10 minutes is adequate for impregnation, although more rapid impregnation, such as obtained in a size press on a paper machine, or longer periods of impregnation may be employed without adverse effects.

While an aqueous treating solution is the most practical, other types of evaporable solvents or carriers can be substituted for Water. In some cases the cellulose material may be treated with the pure active ingredients, without solution, but for most purposes a treating solution is used.

The concentration of the active ingredients used in the solution may vary considerably depending on the end use of the cellulose material and the method of application. Although it is somewhat more difficult to achieve the desired impregnation from very dilute solutions as compared to more concentrated solutions, solutions containing as little as 0.5% active ingredients have been used. Generally, solutions containing from 2 to 10% active ingredients are employed with about 7.5% being preferred.

The weight relationship between the diglycolamine and pentaerythritol is not particularly critical. It has been found that the diglycolamine can be used in the weight ratio of 1:1 to 5:1 with respect to the pentaerythritol.

After the treating solution has been applied to the cellulose material for the desired period of time to provide adequate impregnation, the excess solution is removed or drained from the cellulose and the cellulose is either dried at room temperature or at a suitable elevated temperature to evaporate the water or other carrier, if such is used.

Generally, the greater the amount of active ingredients present, the more th cellulose fibers are stablized. Because the active ingredients may be used in varying proportions in the treating solution, it is difficult to provide a meaningful quantitative indication of the degree of stabilization obtained. When using diglycolamine alone, it has been found that substantial improvement in thermal stability is obtained when the diglycolamine is present in an amount as small as 1% by weight of the cellulose dry fiber base. Similarly, improvement in the thermal stability has been achieved by using as little as 0.25% by weight of the pentaerythritol in combination with the diglycolamine. This concentration of active ingredients in the dried product corresponds generally to 0.1 to 3.0% by weight of nitrogen.

The cellulose fibers treated with diglycolamine or with the combination of diglycolamine and pentaerythritol have greatly improved stability enabling the cellulose fiber material to withstand the deteriorating action of heat over extended periods of time. In addition, when the cellulose fiber material is to be used as an insulating material in electrical apparatus and immersed in transformer oil or other dielectrics, it has been found that the treated paper will not discolor or deteriorate the transformer oil.

' The following examples illustrate the processes of the invention.

EXAMPLE NO. 1

A Sample A was prepared by immersing a sheet of kraft paper 8" x 8" and 5 mils thick in an aqueous solution containing 5% diglycolamine and water until the sheet of paper had become thoroughly impregnated. This sheet was allowed to air dry at room temperature. The impregnated sheet of paper was then placed in a glass tube containing insulated copper wire 21" long (16 gauge Formvar) and copper foil (14" X 1" x .002). This paper-copper system was placed in an oven at for 16 hours. During this time a pressure of 0.1 mm. Was maintained. The evacuated tube containing this paper was filled under vacuum with an inhibited transformer oil, leaving an air space in the tube of approximately 15% of the total volume. The air space was then filled to one atmosphere of pressure with dry air. The tube was then sealed 01f with an oxygen gas torch. The sealed glass tube was then placed in an oven at 17 C. for 5 days. At the completion of the aging period, the tube was opened and the physical properties of the paper were determined along with a chemical analysis for water and acid in the oil. These properties are compared with an untreated paper, Sample C, aged under the same conditions.

EXAMPLE NO. 2.

A Sample B was prepared by immersing a sheet of kraft paper 8" x 8" andS mils thick in an aqueous solution containing 2.5% diglycolamine and 2.5% pentaerythritol until the sheet was thoroughly impregnated. It was then allowed to dry at room temperature. The impregnated sheet of paper was then placed in a glass tube containing an insulated copper wire 21" long (16 gauge Formvar coated) and copper foil (14" x 1" x 0.002"). This papercopper system was placed in an oven at 135 C. for 16 hours. During this time a pressure of 0.1 mm. was maintained. The evacuated tube containing this paper was filled under vacuum with an inhibited transformer oil leaving an air space in the tube of approximately 15% of the total volume. The air space was then filled to one atmosphere pressure with dry air. The tube was then sealed off with an oxygen gas torch. The sealed tube was then placed in an aging oven at 170 C. for 5 days. At the completion of the aging period, the paper was removed, its physical properties determined and the oil was analyzed for acid and water. Properties were compared with those of an untreated aged paper, Sample C.

The results of the tests are shown below.

In the above tests, the tensile strength, toughness, tear strength and burst strength of the Samples A, B and C were compared with the similar properties of untreated and unaged kraft paper. The tensile strength, after aging, of Sample A, which contained 2.5% diglycolamine, was 93% of that of untreated and unaged paper, while the tensile strength of the aged Sample B, containing the combination of diglycolamine and pentaerythritol was 81% of untreated and unaged paper. In contrast to this, the tensile strength of the untreated and aged paper, Sample C, was only 70% of the original unaged kraft paper.

The toughness, tear strength, burst strength and folding ability of the Samples A and B, treated with diglycolamine, and diglycolamine and pentaerythritol, respectively, were also substantially improved over those properties of Sample C which was the untreated aged paper.

These tests show the definite improvement in physical properties of the cellulose material after aging at elevated temperatures brought about by the use of diglycolamine and the combination of diglycolamine and pentaerythritol.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.

We claim:

1. A method of treating cellulose fiber material to increase its physical properties at elevated temperatures, comprising the steps of impregnating the cellulose fibers with the combination of diglycolamine and pentaerythritol, said diglycolamine being present in a weight radio of 1:1 to 5:1 with respect to the pentaerythritol.

2. A method of increasing the thermal stability of cellulose fiber material to be used in an electrical apparatus, comprising the steps of reacting the cellulose fibers with the combination of diglycolamine and pentaerythritol to provide a dry fiber material having a nitrogen content in the range of 0.1 to 3% by weight.

3. A method of increasing the thermal stability of cellulose fiber material, comprising the steps of impregnating the fibers with an aqueous solution of diglycolamine and pentaerythritol, said diglycolamine being present in a weight ratio of 1:1 to 5 :1 with respect to the pentaerythritol, and thereafter evaporating the water to provide a dried product having from 0.1 to 3% by weight of nitrogen.

4. The method of claim 3 in which the aqueous solution contains from 2 to 10% by weight of said diglycolamine and pentaerythritol.

5. A cellulose fiber product having improved physical properties at elevated temperatures, comprising a cellulose fiber base impregnated with the combination of diglycolamine and pentaerythritol, said cellulose base containing from 0.1 to 3% by weight of nitrogen.

6. A cellulose fiber product having improved physical properties at elevated temperatures, comprising a cellulose fiber base impregnated with the combination of diglycolamine and pentaerythritol, said diglycolamine being present in a weight ratio of 1:1 to 5:1 with respect to pentaerythritol, said cellulose base containing from 0.1 to 3% by weight of nitrogen.

7. The structure of claim 6 in which said diglycolamine is present in an amount greater than 1% by weight of the cellulose base and said pentaerythritol is present in an amount greater than 0.25% by weight of the cellulose base.

References Cited UNITED STATES PATENTS 1,594,982 8/1926 Somerville 317259 3,316,178 4/1967 Millington 25263.7

NORMAN G. TORCHIN, Primary Examiner.

J. CANNON, Assistant Examiner. 

1. A METHOD OF TREATING CELLULOSE FIBER MATERIAL TO INCREASE ITS PHYSICAL PROPERTIES AT ELEVATED TEMPERATURES, COMPRISING THE STEPS OF IMPREGNATING THE CELLULOSE FIBERS WITH THE COMBINATION OF DIGLYCOLAMINE AND PENTAERYTHRITOL, SAID DIGLYCOLAMINE BEING PRESENT IN A WEIGHT RADIO OF 1:1 TO 5:1 WITH RESPECT TO THE PENTAERYTHRITOL. 